-Wall -Werror

| Patch pushed. I've plumped for option a. It's better structuring, as b
| would involve moving more code from Gregorian.hs to Days.hs.
|| The concern Henning raised shouldn't apply, as both modules are hidden
| and re-exported by Data.Time.Calendar. I've also fixed two other modules
| with the same issue (again, also hidden).
Thanks. It doesn't matter whether they are hidden or not --- their instances are visible regardless in Haskell. So those interface files will be read any time you compile a module that depends on a module in the time package. But that's not a terribly big deal.
Simon